The present invention relates to a high-speed electrical signal transmission line employed as a twisted pair cable or a coaxial cable. This type of transmission line often employs a porous polytetrafluoroethylene resin as its dielectric. Examples of a trasmission line which employs a porous polytetrafluoroethylene resin as its dielectric include that in the "Method of Manufacturing Electric Wires Coated with Tetrafluoroethylene Polymer" disclosed in the specification of Japanese Patent Publication No. 21,809/1982. That invention has an arrangement in which, in order to prevent any increase in specific gravity of a porous polytetrafluoroethylene resin layer, an unsintered polytetrafluoroethylene resin tape which has previously been rolled and compressed so that its specific gravity is in the range of 1.85 to 2.2 is wound around the outer periphery of a conductor and is then sintered at a temperature of 327.degree. C. to 400.degree. C. That method, however, still does not completely prevent an increase in the specific gravity of the resin layer. When a conductor having such a resin tape wound thereon is employed as a transmission line, the signal propagation delay time is around 4.0 ns./m, and it is not possible to increase the transmission speed substantially.
In view of the above circumstances, the present inventors have found as the result of zealous examination that the specific gravity of a porous dielectric made of a plastic resin starts to increase when heated at a temperature exceeding 60% of the melting temperature of the resin, and there is virtually no increase in specific gravity of the porous dielectric when heated at a temperature of 60% or less of the melting temperature of the resin.
Accordingly, an object of the present invention is to provide a transmission line in which the specific gravity of a porous plastic dielectric is maintained at 0.5 or less for the purpose of stably providing a transmission characteristic which makes it possible to attain a cable having a signal propagation delay time of about 3.5 ns./m.